Bill,
What do the road tests show for the rear moldings on non-RS cars?
Ads are a poor source of data, btw. The cars are often not indicative of production.
Really had to do some digging. Here's a '68 Nickey Camaro promo photo. No idea of the source; Nickey closed in the early '70s. Looks like the rear molding ends near the rocker spear. Based on the stripe the car was built prior to January 1968.
Dug out copies of '68 Camaro body trim from the illustration index in the 1/1/68 P & A book. Only one rear wheel well molding is listed; 7741686 & 7741687 [L & R]. At the front, 4 part numbers are listed designated 'RS' and 'exc RS'.
Conclusion: I stand corrected. It appears Chevy intended to and built '68 RS and Z21 Camaros with the clipped rear wheel well molding. Other pics are inconclusive. I did find a feature on a special paint '68 Z/28 in the January 1973 Super Stock. It is a style trim car and has the clipped rear molding.
FWIW I did find a Chevy promotional pic of a '68 Camaro and it shows a full-length molding.
Looks dumb to me but apparently built that way.